There are a number of industrial processes wherein a liquid and a gas are brought into direct contact with each other for the purpose of effecting a transfer of heat from one medium to the other. The efficiency with which the heat transfer process occurs is dependent on the amount of liquid surface area that comes into direct contact with the gas. Most of the apparatus specifically designed for this type of process employs some physical means whose primary purpose is to promote the generation of liquid surface. This is accomplished by either promoting the generation of liquid droplets by means of a splash bar type fill assembly or by promoting the generation of thin liquid films on the surface of a cellular structure designed in such a way that the gas may flow through the passages of the cellular structure. The second type of media is commonly called a film type fill assembly. Examples of film type fill assemblies are shown in U.S. Pat. No. 2,809,813 patented on Oct. 15, 1957, U.S. Pat. No. 2,986,379 patented on May 30, 1961, U.S. Pat. No. 3,262,682 patented on July 26, 1966, U.S. Pat. No. 3,272,484 patented on Sept. 13, 1966, and U.S. Pat. No. 4,117,049 patented on Sept. 26, 1978.
U.S. Pat. Nos. 2,809,818 and 2,985,379 show film type packings composed of a plurality of thin sheets where adjacent sheets are secured by means of a suitable adhesive. The cellular structure is fabricated by arranging the sheets such that a flat sheet is adjacent to a corrugated sheet alternately throughout the structure thereby creating vertical passageways. Liquid is distributed or sprayed over the top of the cellular structure and flows downward through the passages adhering to the passage walls in the form of thin films. Concurrently the gas is forced upwardly through said passages in countercurrent flow relationship to liquid film flow. These designs are limited to counterflow arrangements and further their heat transfer efficiency is limited primarily because there is no means inherent in these designs to promote even distribution and uniform thickness and flow of the liquid film. Further there is nothing inherent in these designs to promote turbulence and mixing of the main body of gas flowing through the passages and gas stratification limits heat transfer efficiency.
U.S. Pat. No. 3,262,682 overcomes these limitations to some extent. All sheets are corrugated and adjacent sheets are oriented and connected such that the corrugations extend at an oblique angle relative to a horizontal plane with every second layer having its corrugations oriented obliquely in one direction with adjacent and subsequent second layers extending obliquely in the opposite direction. This cellular configuration creates passageways of constantly varying cross section and the passageways in both the horizontal and vertical directions have a serpentine-like shape. These features promote uniformity in the distribution and thickness of liquid films and causes the gas to mix thoroughly as it travels through the serpentine passages. Further, this cellular structure may be used in both counterflow and crossflow gas-liquid flow arrangements since gas may be directed to enter the passageways either from the bottom or side of the cellular structure respectively.
Further improvements in the flow and distribution of liquid in cellular film type packing designs are taught in U.S. Pat. Nos. 3,272,484 and 4,117,049. U.S. Pat. No. 3,272,484 shows cellular structures where the parallel passageways have either a hexagonal or triangular cross section with passages aligned in the generally horizontal direction of gas flow. In order to provide for the downward passage of liquid in direction transverse to the axis of the passages, walls of each passageway are apertured at spaced locations along the length thereof which provides liquid communication with the adjacent row of passages therebelow and succeeding rows of apertures are staggered so that liquid will be caused to distribute itself in the form of films on passageway walls during its downwardly directed flow through the cellular structure. U.S. Pat. No. 4,117,049 shows yet another means for redistributing liquid and gas within passageways created by means of a plurality of connected sheets whereby each adjacent, generally horizontal sheet pair is connected by means of accordian-like side walls thereby creating a cell with each sheet apertured on one side to permit communication of fluids from one cell to those immediately above and below. The apertures on an adjacent sheet are located on the opposite side of the cell. Said apertures act as staggered inlet and outlet ports for both liquid and gas flow in the cell created by two successive parallel sheets and the connecting, accordian-like side walls. By connecting a number of such cells vertically, a cellular column is formed with ports on opposite sides of each adjacent cell, and an independent zigzag fluid flow path is created in each cellular column between the top and bottom of the cellular structure.
Among the problems associated with film type packings is that the gas is required to flow through passages which are relatively small in cross section and further the gas is often required to follow a tortuous path while within the confines of the cellular structure. These factors result in relatively high resistance to flow of the gas stream which results in higher energy usage by the gas moving device of the apparatus. Consequently, the application of film type packings is limited to smaller systems or large systems where only a few feet of film type packing is required.
A further limitation is that the quantity of liquid per unit area must necessarily be limited since otherwise the flowing films of liquid on sheet surfaces become relatively thick thereby limiting the liquid-gas contact area, impeding heat transfer efficiency. These thick liquid films will also restrict the area of the gas flow passages thereby further increasing resistance to gas flow. Yet another limitation is that the cellular passages, being necessarily small in an effort to obtain maximum liquid surface area in a given volume, can easily plug up if any solid foreign matter or chemical substance with a tendency to precipitate is present in either the liquid or gas. Yet another limitation is that the sheets from which film pack structures are formed are necessarily thin for economic reasons and are easily crushed or damaged particularly along the edges. Shipping costs are also high since these relatively light weight structures consume substantial volume when shipped in assembled form and field assembly is usually prohibitive from a cost standpoint. Generally, film type packings will have high heat transfer capabilities per unit volume, but the limitations described above, coupled with high unit costs, limit their application in practice.
Splash bar type fill assemblies generally overcome the limitations of film type fill structures particularly noted above. These designs consist of a plurality of splash bars, supported in a frame or grid wherein said splash bars are placed in a horizontal plane in parallel, spaced-apart relationship in multiple rows wherein the splash bars in adjacent rows above and below are placed in staggered, offset relationship relative to each other. In direct contact heat exchange apparatus where the intended flow of the gas is generally in crossflow relationship with the flow of liquid, two general orientations of splash bar fill assemblies are known. The most common type consists of a matrix of splash bars as described above wherein the bars are oriented such that gas flow is generally perpendicular to the longitudinal axis of the individual bars. The vertical dimension of bars disposed in this orientation presents an obstruction to gas flow and of necessity splash bars designed primarily for this orientation should have a relatively low and aerodynamically efficient profile in the transverse direction to minimize the resistance to gas flow thereby minimizing the amount of energy required to induce gas flow through the apparatus. Prior art examples of splash bar designs oriented with the longitudinal axis of the bar perpendicular to gas flow have transverse shapes which generally demonstrate either a compromise in the strength of the splash bar, or project an aerodynamically inefficient profile in the gas flow direction are shown in U.S. Pat. No. 3,389,895 issued June 25, 1963, U.S. Pat. No. 3,468,521 issued Sept. 23, 1963 and U.S. Pat. No. 3,647,191 issued on Mar. 7, 1972.
U.S. Pat. No. 3,389,895 shows splash bars intended for the above-described orientation with open base triangular and rectangular transverse profiles and perforate surfaces, both of which present large and aerodynamically inefficient projected areas in the direction of gas flow. The same is true of the M-shaped open base profile shown in U.S. Pat. No. 3,647,191. In addition to the higher resistance to gas flow, these designs also have limitations in that gas deflected by the blunt projected area is directed away from at least part of the major splash surface of the profile and intimate mixing of gas and liquid is thereby impeded to some extent. Further, these profiles have only a small bearing surface area at points where bars rest on supporting grids which results in excessive wear at these points with a substantial shortening of the useful life of the splash bar since the profile eventually wears through at these contact points.
U.S. Pat. No. 3,468,521 shows a similarly oriented splash bar consisting of a perforate strip having an elongated, convex leading edge where the upper surface slopes downwardly, terminating at the convex leading edge. While this profile presents a generally more favorable profile in the direction of gas flow, it still has the limited bearing surface at grid support points noted above. Further, the sloped upper surface will cause liquid impinging on said surface to collect and flow forward in streams unless the perforate surface has relatively large holes to allow liquid to pass therethrough. In practice larger holes are used to avoid this. However, the larger holes result in less effective mechanically induced droplet fragmentation of liquid forced through the perforate surfaces thereby diminishing the extent of droplet generation and dispersion that otherwise might be achieved and diminishing the overall heat transfer efficiency.
Other splash bar configurations specifically designed for an orientation such that the gas flow is parallel to the longitudinal axis overcomes some of the limitations noted above and generally offer less resistance to gas flow. Typical examples are found in U.S. Pat. No. 2,497,389 issued Feb. 14, 1950, U.S. Pat. No. 3,758,088 issued Sept. 11, 1977, U.S. Pat. No. 4,020,130 issued Apr. 26, 1977, U.S. Pat. No. 4,133,851 issued Jan. 9, 1979 and U.S. Pat. No. 4,181,691 issued Jan. 1, 1980.
U.S. Pat. Nos. 2,497,389 and 3,758,088 show planar and non-planar sine wave fill members respectively, oriented with the longitudinal axis of the fill member parallel to the direction of gas flow. These profiles have no perforate openings and thus lack the ability to fragment liquid by shearing as liquid passes through the perforate surfaces embodied in other designs.
Designs with perforate surface sections such as those shown in U.S. Pat. Nos. 4,020,130, 4,133,851 and 4,181,591 overcome this limitation, but still do not provide the advantages of the present invention. In particular, the profiles shown in U.S. Pat. Nos. 4,020,130 and 4,181,691 incorporate horizontal perforate surfaces which provide the main means for splash and mechanically induced liquid fragmentation and dispersion.
The designs taught in U.S. Pat. No. 4,020,130 consist of two horizontal perforate strips connected by means of one vertical perforate strip where the free edges of said horizontal perforate strips terminate in a small lip or projection to provide additional lateral stability and strength to the profile. In order to obtain adequate structural strength and stability of the profile it is necessary to limit the extent of the transverse dimension of the horizonal perforate surfaces which provide the main means of liquid splash and fragmentation. This reduces the overall liquid dispersion effectiveness that might otherwise be obtained since a limited horizontal perforate surface is present to disperse and fragment liquid on a given splash bar. Further, the vertical strip connecting said horizontal surfaces must extend vertically a significant distance, again to achieve reasonable structural strength and the profiles taught must be supported within the confines of a grid support system containing means for restricting movement of the top, bottom and lateral extents of the profile. These dimensional limitations, which exist primarily for practical structural reasons, limit the application of these profiles to crossflow liquid-gas flow relationships where the gas flows parallel to the longitudinal axis of the profile. If these profiles are oriented with gas flow perpendicular to the longitudinal axis of the profile the vertical connecting element presents an extensive, and blunt projection in the direction of gas flow thereby creating a higher resistance to gas flow than desired. The splash bar profile of U.S. Pat. No. 4,181,691 overcomes the problem of having to limit the transverse extent of the horizontal perforate surface by incorporating a vertical strip at each of the transverse edges of the horizontal perforate strip. Said strips must again project vertically upward a significant distance to obtain the desired structural strength of the profile. Thus this profile is again only suitable for crossflow orientation with gas flow parallel to the longitudinal axis of the splash bar for reasons explained above. The splash bar profiles taught in both of these patents have a further and very significant limitation in that the vertical strips and edge lips extend upward from at least one of the major horizontal liquid splash and dispersion surfaces and an open U-shaped channel is presented to falling liquid which can cause a portion of the liquid impacting the horizontal surface to be trapped in the trough thus formed. While this trapped liquid will eventually drain through the perforate surface, the formation of thick liquid films on the horizontal surface will diminish the effectiveness of splash induced liquid fragmentation on said surfaces. In addition, some of this liquid will migrate down the longitudinal axis of the splash bar in the direction of gas flow. Further, liquid passing through the holes and/or overflowing the vertical side strips will continue its fall in the form of heavy streams as opposed to droplets. These factors have a negative impact on the uniformity of water distribution throughout the splash bar matrix area and reduce the liquid contact surface area that might otherwise be achieved.
The splash bar profiles taught in U.S. Pat. No. 4,133,851 overcome the trapped liquid and flow uniformity problems noted above by incorporating only one vertical perforate or imperforate strip in the profile design and by positioning said vertical strip parallel to the longitudinal axis of the bar and positioned either at center or at a single transverse edge of the horizontal, perforate surface of the bar. Further the edges of both the vertical and horizontal strips include a bevel or skirt whose purpose is to direct any accumulating water toward the horizontal splash surface of said bar or horizontal surface of other splash bars located below in the splash bar assembly matrix which are positioned in lateral offset relationship. Vertical strips and the beveled skirts provide some functional advantages as noted above but also must be relied upon to provide structural strength and rigidity to the splash bar. The structural limitations of U.S. Pat. No. 4,020,130 as relates to practical limits on the lateral extent of the horizontal perforate surfaces and subsequent limit of liquid dispersion capability still exist with this teaching as does the necessity to limit its application to an orientation wherein gas flow is parallel to the longitudinal axis of the splash bar for reasons described above. Thus while all three of these patents offer some improvement over prior art, they still fail to provide a splash bar configuration that can provide the ultimate combination of maximum liquid splash and dispersion characteristics, and hence maximum heat transfer, uniform liquid dispersion and distribution throughout the entire splash bar fill matrix area, minimum resistance to gas flow independent of orientation of the splash bar relative to gas flow direction and exceptional strength without limiting the dimensions of the splash bar in a way that will either detract from the performance characteristics of the splash bar or ultimately result in the use of more material to achieve the same results. The teachings of the present invention overcome these limitations.
While the differences in the various splash bar designs found in the prior art may appear subtle, those skilled in the art will recognize that they are never the less critical in terms of the ability of a given profile to generate maximum liquid contact surface area and the overall heat transfer capability of the splash bar matrix assembly as well as their effect on the overall energy requirements of the gas moving device of the apparatus. Further, the strength, durability and cost are major considerations that cannot be overlooked. Clearly, much is yet to be done to obtain the ultimate functional relationship between gas and liquid in a splash bar design and assembly matrix while at the same time minimizing resistance to gas flow, providing adequate strength and durability and obtaining liquid flow uniformity and increasing liquid retention time throughout the splash bar assembly matrix.